Raman spectroscopy has recently enjoyed widespread use in the fields of material science, chemical engineering, pharmacology, environmental technologies, chemical analysis, and process monitoring, mostly due to the development of less expensive semiconductor lasers. In many applications, the employed sensors are typically coupled to a spectrometer.
One general limitation for application of Raman spectroscopy in the aforementioned technical fields is that a fluorescence background or a broadband background scattering is frequently excited in addition to the desired Raman signals. Especially biological samples tend to fluoresce when the Raman effect is excited in the visible spectral range, which may completely obscure the Raman spectra. Although practically no fluorescence is produced when the Raman effect is excited with radiation in the far-infrared spectral range, the intensity of the Raman scattered radiation decreases with the fourth power of the absolute wave number, so that the optical spectrum analyzer must have a significantly higher sensitivity, which increases the complexity of the system.
Another problem is that with CCD detectors, the baseline has a characteristic structure, also referred to as fixed pattern. The fixed pattern is an unmovable interfering structure which is superimposed on the images of CCD cameras or CCD sensors. The fixed pattern masks the weak Raman signals (when using CCD-based detectors) and limits the attainable sensitivity. Conventional methods corrected this effect by requiring the measurement of a dark or null spectrum. However, even with this correction, the fix pattern can frequently not be eliminated sufficiently, because the measurement is performed in another intensity range and does not adequately take into consideration the physical nature of the fixed pattern.
Fluorescence suppression as well as background corrections have been investigated in Raman spectroscopy for many ways. For example, fluorescence in the spectrum can be eliminated by rapid gating, i.e., by taking advantage of the effect that the fluorescence response is slow compared to the Raman effect. However, this requires complex experimental setups with pulsed lasers, as disclosed, for example, by P. Matousek et al. “Fluorescence suppression in resonance Raman spectroscopy using a high-performance picosecond Kerr gate”, J. Raman Spectroscopy 2001, 32, 983-988.
In addition, A. P. Sheve et al., Appl. Spectroscopy 1992, 46, 707, disclose that the fluorescence background can be corrected or even eliminated by using two laser wavelengths which are wavelength-shifted relative to one another. Sheve et al. use as a light source a Ti:Sapphire laser emitting at two frequency-shifted wavelengths produced with a diffractive element. However, disadvantageously, this setup is also rather complex.
Conventional methods and devices disadvantageously require complex equipment to attain adequate sensitivity for generating and detecting Raman spectra.